Diatoms are generally the dominant contributors to the Arctic Ocean spring
bloom, which is a key event in regional food webs in terms of capacity for
secondary production and organic matter export. Dissolved silicic acid is an
obligate nutrient for diatoms and has been declining in the European Arctic
since the early 1990s. The lack of regional silicon cycling information
precludes understanding the consequences of such changes for diatom
productivity during the Arctic spring bloom. This study communicates the
results from a cruise in the European Arctic around Svalbard, which reports the
first concurrent data on biogenic silica production and export, export of
diatom cells, the degree of kinetic limitation by ambient silicic acid, and
diatom contribution to primary production. Regional biogenic silica
production rates were significantly lower than those achievable in the
Southern Ocean and silicic acid concentration limited the biogenic silica
production rate in 95 % of samples. Compared to diatoms in the Atlantic
subtropical gyre, regional diatoms are less adapted for silicic acid uptake
at low concentration, and at some stations during the present study, silicon
kinetic limitation may have been intense enough to limit diatom growth. Thus,
silicic acid can play a critical role in diatom spring bloom dynamics. The
diatom contribution to primary production was variable, ranging from
<10 % to ∼100 % depending on the bloom stage and
phytoplankton composition. While there was agreement with previous studies
regarding the export rate of diatom cells, we observed significantly elevated
biogenic silica export. Such a discrepancy can be resolved if a higher
fraction of the diatom material exported during our study was modified by
zooplankton grazers. This study provides the most direct evidence to date
suggesting the important coupling of the silicon and carbon cycles during the
spring bloom in the European Arctic.

Diatoms and the haptophyte Phaeocystis are the dominant contributors to the Arctic
Ocean spring bloom, a cornerstone event supplying much of the annual net
community production (Rat'kova and Wassmann, 2002; Vaquer-Sunyer et al.,
2013; Wassmann et al., 1999) that fuels Arctic food webs (Degerlund and
Eilertsen, 2010, and references therein). Hydrographic and chemical changes
in the Arctic water column are expected in the future, but whether these
will alter diatoms' contribution to spring primary production and organic
matter export remains uncertain. Some studies predict reduction in ice cover
will enhance the spring bloom due to increased light availability (Arrigo et
al., 2008), while others predict lower productivity driven by increased
stratification and reduced nutrient supply (Schourup-Kristensen et al., 2018;
Tremblay and Gagnon, 2009). Additionally, models predict that warming will
lead to a shift from a diatom-dominated bloom to one increasingly dominated
by flagellates and picoautotrophs, which has been observed in certain
sectors of the Arctic (Li et al., 2009; Lasternas and Agustí, 2010).
Because the spring diatom bloom is arguably the single most important
productivity event for the Arctic Ocean ecosystem (Degerlund and Eilertsen,
2010; Holding et al., 2015; Vaquer-Sunyer et al., 2013), understanding how
diatoms' ecological and biogeochemical importance changes in response to
system-wide physical or chemical shifts is important to predict future food web
alterations. Diatoms have an obligate requirement for silicon; therefore
understanding regional silicon cycling can provide insights into diatoms'
activity. However, there is a current knowledge gap in regional silicon
cycling, which precludes robust assessments of the spring bloom in future
scenarios, e.g., Tréguer et al. (2018).

Diatom production is dependent on the availability of dissolved silicic acid
(Si(OH)4), which they use to build their shells of biogenic silica
(bSiO2). [Si(OH)4] has been observed to be low (<5µM)
in the Norwegian Seas and declining over time (Rey, 2012). A more recent
analysis demonstrated a decline in pre-bloom [Si(OH)4] concentrations
by 1–2 µM across the North Atlantic subpolar and polar regions over
the last 25 years (Hátún et al., 2017). This is in stark contrast to
the 10–60 µM [Si(OH)4] observed in the surface waters of the
Southern Ocean and the marginal ice zone around Antarctica (Nelson and
Gordon, 1982; Brzezinski et al., 2001), where [Si(OH)4] is unlikely to
limit diatom growth unless iron is replete and stimulates exceptional
blooms which consume Si, or assemblages are highly inefficient for Si uptake
(Brzezinski et al., 2001). Additionally, the stoichiometry of Si(OH)4 availability
relative to nitrate (Si:N<1) in the source waters, which fuel the
spring bloom in most of the North Atlantic and European polar seas, suggests
that during a bloom cycle diatoms may experience Si limitation prior to N
limitation, especially if diatoms consumed Si and N in near-equal quantities
as in other diatom bloom regions (Brzezinski et al., 1997; Brzezinski, 1985;
Dugdale et al., 1995).

Compared to the Southern Ocean, there is a paucity of field Si-cycling
studies in the European Arctic. Reports of diatom silica production are only
available from the subarctic northeast Atlantic near ∼60∘ N, e.g., between Iceland and Scotland (Allen et al., 2005;
Brown et al., 2003), in Oslofjorden (Kristiansen et al., 2000), and in Baffin Bay (limited data; Hoppe et al., 2018; Tremblay et al., 2002); these
previous studies are in zones with higher Si(OH)4 availability than in
the European Arctic. Other studies have reported standing stocks of
bSiO2 and export in Oslofjorden or the European Arctic, e.g., Svalbard
vicinity, Laptev Sea (Hodal et al., 2012; Heiskanen and Keck, 1996; Paasche
and Ostergren, 1980; Lalande et al., 2016, 2013), but none
have concurrent measurements of bSiO2 production. Indeed, in the last
major review of the global marine silicon cycle, Tréguer and De La Rocha
(2013) reported no studies with published bSiO2 production data from the Arctic.

Currently, we lack a baseline understanding on diatom Si-cycling in the
European Arctic and broader high-latitude North Atlantic region. And while
models in the Barents Sea use Si as a possible limiting nutrient (Wassmann
et al., 2006; Slagstad and Støle-Hansen, 1991), there are no field data
to ground truth the modeled parameters governing diatom Si uptake. Thus,
there is no contextual understanding to determine the consequences of the
observed changes in regional [Si(OH)4] since the 1990s and whether these
affect spring bloom dynamics. This study communicates the results from a
cruise in the European Arctic around Svalbard reporting the first concurrent
data sets on regional bSiO2 production and export, the export of diatom
cells, and the degree of kinetic limitation by ambient [Si(OH)4].
Additionally, coupling bSiO2 production rates with contemporaneous
primary production measurements provides an independent assessment for the
diatom contribution to system primary production.

2.1 Region and sampling

This study was conducted onboard the RV Helmer Hanssen between 17 and 29 May 2016 as part of
the broader project, ARCEx – The Research Centre for ARCtic Petroleum
Exploration (http://www.arcex.no/, last access: 29 October 2018). The main goal of this
cruise was to study the pelagic and benthic ecosystem during the Arctic
spring bloom around Svalbard and in the northern Barents Sea at stations
which are influenced by various water masses. The cruise started in the southwestern
fjords influenced by relatively warm Atlantic water, then transited east of
Svalbard toward more Arctic-influenced water (Fig. 1 blue arrow) before
turning south towards stations near the Polar Front and south of the Polar
Front in Atlantic-influenced water (Fig. 1 red arrows).

Vertical profiles with a CTD were conducted at all stations. Hydrocasts were
conducted using a Seabird Electronics 911 plus CTD with an oxygen sensor,
fluorometer, turbidity meter, and PAR sensor (Biospherical/LI-CORR, SN 1060).
The CTD was surrounded by a rosette with 12 5 L Niskin bottles. At
two stations, Edgeøya and Hinlopen, only surface samples were collected
(no vertical profiles with ancillary measurements, Fig. 1). Water was
sampled from the rosette at depths within the upper 40 m (i.e., the extent of
the photic layer); for any incubation described below, the approximate
irradiance at the sample depth during collection was mimicked by placing
incubation bottles into a bag made of neutral-density screen. Incubation
bags were placed in a onboard acrylic incubator cooled with continuously
flowing surface seawater. At Hinlopen, a block of ice was collected by hand
within ∼10m of the vessel and allowed to thaw in a shaded
container for 24 h at ambient air temperature. After thawing, the melted
solution was homogenized and treated like a water sample for measurement of
biomass and rates.

Four sediment trap arrays were deployed and collected particulate material for 19 and
23 h, depending on location. Arrays in
van Mijenfjorden and Hornsund were anchored to the bottom (60 and 130 m,
respectively), whereas the other two arrays (Erik Eriksenstretet,
260 m
bottom depth; Polar Front, 290 m bottom depth) were quasi-Lagrangian and
drifted between 14 and 16 km during the deployment. During the Erik
Eriksenstretet deployment, the array was anchored to an ice floe. Arrays
included sediment trap cylinders (72 mm internal diameter ×450mm
length,
∼1.8L volume; KC Denmark) at three (van Mijenfjorden) to seven
(Atlantic Station) depths between 20 and 150–200 m, based on bathymetry.
After recovery, trap contents were pooled and subsampled for bSiO2 and
phytoplankton taxonomy.

2.2 Standing stock measurements

A suite of macronutrients were analyzed at all stations except Hinlopen
(just Si(OH)4). Water was sampled directly from the rosette, filtered
(0.7 µm pore size), and immediately frozen. In the laboratory,
nutrients were analyzed using a Flow Solution IV analyzer (OI Analytical,
USA) and calibrated with reference seawater (Ocean Scientific International
Ltd., UK). Detection limits for [NO3+NO2] and [Si(OH)4]
were 0.02 and 0.07 (µM), respectively. No ammonium was measured. To
avoid artefacts with prolonged freezing (Clementson and Wayte, 1992;
Macdonald et al., 1986), samples were analyzed within 4 months of collection
and standard practices were used (e.g., prolonged thawing of Si(OH)4
samples to allow depolymerization, three parallels measured). The median
coefficient of variation among parallels was 5 % for [NO3+NO2]
and [PO4], 2 % for [Si(OH)4], and 9 % for
[NO2]; higher coefficient of variation was observed when the absolute
concentrations were low, e.g., <0.1µM. Reproducibility was
sufficient, and no parallels were excluded. Phosphate was analyzed, but
N:P
ratios for nutrients were, on average, 8 among all stations, suggesting that
N was likely more important than P for potentially limiting primary
production. These phosphate data (0.1–0.6 µM in the upper 50 m) are
not discussed.

Samples for biogenic particulates and phytoplankton community composition
were taken directly from the rosette and sediment traps. For bSiO2
samples, 600 mL of seawater was collected from the rosette, filtered through
a 1.2 µm polycarbonate filter (Millipore); for sediment trap
material, less volume was necessary (e.g., 50–100 mL). Most bSiO2
protocols use a 0.6 µm filter cutoff, e.g., Lalande et al. (2016);
however, given the magnitude bSiO2 quantified and the size range for
regional diatoms we are confident that there was no meaningful systematic
underestimate. After filtration, all samples were dried at 60 ∘C
and stored until laboratory analysis using an alkaline digestion in Teflon
tubes (Krause et al., 2009). For Chl a, water-column and sediment-trap samples
were collected similarly, filtered on Whatman GF/F (0.7 µm pore
size), and immediately frozen (−20∘C). In the laboratory, Chl a was
extracted in 5 mL methanol in the dark at room temperature for 12 h. The
solution was quantified using a Turner Designs 10 AU fluorometer, calibrated
with Chl a standard (Sigma C6144), before and after adding two drops of 5 %
HCl (Holm-Hansen and Riemann, 1978). Phytoplankton taxonomy and abundance
samples were collected in 200 mL brown glass bottles from both the water
column and sediment traps, immediately fixed with an aldehyde mixture of
hexamethylenetetramine-buffered formaldehyde and glutaraldehyde at 0.1 % and
1 % final concentration, respectively, as suggested by Tsuji and Yanagita
(1981), and stored cool (5 ∘C) and dark. Samples were analyzed with
an inverted epifluorescence microscope (Nikon TE300 and Ti-S, Japan), using
the Utermöhl (1958) method, in a service laboratory for diatom taxonomy
(>90 individual genera or species categories were identified) and
abundance at the Institute of Oceanology Polish Academy of Science.

2.3 Rate measurements

Biogenic silica production was measured using the radioisotope tracer
32Si. Approximately 150 or 300 mL samples, depending on the station
biomass, were incubated with 260 Bq of high specific activity
32Si(OH)4 (>20kBq µmol Si−1). After
addition, samples were transported to the onboard incubator and placed in
neutral density screened bags, simulating 50 %, 20 %, and 1 % of
irradiance just below the surface, for 24 h. After incubation, samples
were processed immediately by filtering bottle contents through a 25 mm, 1.2 µm
polycarbonate filter (Millipore) matching bSiO2 filtrations.
Each filter was then placed on a nylon planchette, covered with Mylar when
completely dry, and secured using a nylon ring. Samples were aged into
secular equilibrium between 32Si and its daughter isotope,
32P
(∼120 days). 32Si activity was quantified on a GM
multicounter (Risø National Laboratory, Technical University of Denmark)
as described in Krause et al. (2011). A biomass-specific rate (i.e., Vb)
was determined by normalizing the gross rate (ρ) to the corresponding
[bSiO2] at the same depth of collection using a logistic-growth
approach (Kristiansen et al., 2000; Krause et al., 2011). For bSiO2 and
ρ, values within a profile were integrated throughout the euphotic
zone (i.e., surface to 1 % I0) using a trapezoidal scheme. A
depth-weighted Vb was calculated within the euphotic zone by
integrating Vb and dividing by depth-integrated values (Krause et al.,
2013).

Two methods were used to assess whether ambient silicic acid (Si(OH)4)
limited diatom Si uptake. The 32Si activity additions, incubation
conditions, and sample processing are as described above. At four stations
(Edgeøya, Polar Front, Hinlopen, and Atlantic), eight 300 mL samples
collected at a single depth within the euphotic zone were manipulated to
make an eight-point concentration gradient between ambient and +18.0µM
[Si(OH)4]; the maximum concentration was assumed to saturate
Si uptake. Si uptake has been shown to conform to a rectangular hyperbola
described by the Michaelis–Menten equation:

(1)Vb=VmaxSi(OH)4KS+Si(OH)4,

where Vmax is the maximum specific uptake rate and KS is
half-saturation constant, i.e., concentration where Vb=0.5×Vmax. Data were fit to Eq. (1) using a nonlinear curve fit algorithm
(SigmaPlot 12.3). The second type of experiment used only two points:
ambient and +18.0µM [Si(OH)4]; four-depth profiles were done
at three stations (Bellsund Hula, Hornsunddjupet, Erik Eriksenstretet). The
ratio of Si uptake at +18.0µM [Si(OH)4] to Si uptake at
ambient [Si(OH)4] defines an enhancement (i.e., Enh) statistic. This
two-point approach was conducted at all depths in the euphotic zone; Enh
ratios >1.08 imply kinetic limitation beyond analytical error
given the methodology (Krause et al., 2012).

Table 1Station properties including surface temperature, nutrients and
chlorophyll a (± standard deviation), 20 m integrated biogenic silica stock
(∫bSiO2), production (∫ρ),
depth-weighted specific production (VAVE), 40 m integrated diatom
abundance (∫Diatom), and export of bSiO2 and diatoms at 40 m.
The disparity between the integration depths for bSiO2 standing stock
and diatom abundance reflects the lack of bSiO2 samples to 40 m depth.
Note: Hinlopen (ice) station not included. The Polar Front ∫Diatom is the mean of two profiles.

a Denotes concurrent primary production and biogenic silica
production measurements at one depth.b 25 m depth.c Surface values, no profile taken.

Net primary productivity (PP) was quantified concurrently with biogenic
silica production at six stations at the depth of approximately 50 % of
surface irradiance (Table 1). Carbon uptake rates were measured using a
modification of the 14C uptake method (Steemann Nielsen, 1952). Water
samples were spiked with 0.2 µCi mL−1 of 14C labeled
sodium bicarbonate (Perkin Elmer, USA) and distributed in three clear
plastic bottles and one dark (40 mL each). Subsequently, they were incubated
for 24 h in the deck incubator with a 50 % light reduction mesh. After
incubation, samples were filtered onto 0.2 µm nitrocellulose filters.
The filters were stored frozen (−20∘C) in scintillation vials
with 10 mL EcoLume scintillation liquid (MP Biomedicals LLC, USA) until
further processing. Once on land, the particulate 14C was determined
using a scintillation counter (TriCarb 2900 TR, Perkin Elmer, USA). The
carbon uptake values in the dark were subtracted from the mean of the
triplicate carbon uptake values measured in the light incubations. Using
contemporaneous ρ measurements and PP measurements, the diatom
contribution to PP is estimated as follows:

(2)Diatom % PP=100×ρ×(Si:C)-1PP,

where the Si:C ratio for diatoms can be used from culture values. The most
widely used Si:C ratio is 0.13 (Brzezinski, 1985); however, this study
lacked polar diatom strains. Takeda (1998) grew two polar diatoms at
2 ∘C and in iron-replete media and reported Si:C from 0.10 to 0.18;
however, this was extrapolated based on direct measurement of cellular N and
converting using the Redfield–Ketchum–Richards C:N ratio of 6.6. A more
recent study, Lomas and Krause (2018), reported data on 11 polar diatom
species grown at 2 ∘C with direct measurement of biogenic silica
and particulate organic carbon and nitrogen. For larger diatom species
(>1000µm3 biovolume) these authors observed the
average Si:C was 0.25±0.04 (standard error, SE), with a higher ratio for smaller
species (<1000µm3) 0.32±0.04 (SE). Most of the
diatom assemblage during ARCEx was composed of larger cells; thus, we use
Si:C of 0.25.

Export rates were calculated using the standing stock measurements, length
of deployment, and trap opening area (0.004 m2). These approaches are
common and detailed elsewhere (Wiedmann et al., 2014; Krause et al., 2009).

3.1 Hydrography and spatial patterns

The regional ecosystem around Svalbard is driven by ice dynamics (Sakshaug,
2004). A majority of the southern Svalbard
archipelago had open water 1 week prior to the cruise open water, which was anomalous compared to similar dates in
previous years (e.g., 2014, 2015, ice data archived at http://polarview.met.no/, last access: 29 October 2018).
By the end of the cruise, Svalbard could have
been entirely circled by the vessel, with only open drift ice in the
northeastern region. While 2016 was among the lowest years for total Arctic
sea ice, the ice extent in Svalbard and the Barents Sea is highly dynamic.
Ice edges may be pushed southward into the Barents Sea proper by wind while
areas to the north remain ice-free, e.g., Wassmann et al. (1999) and
references therein.

Spatial patterns in hydrography and nutrients were highly variable. In the
southwestern stations (e.g., fjords and Atlantic-influenced water), the
surface temperature ranged between 1 and 4 ∘C; a similar temperature
was observed in the Atlantic station south of the Polar Front (Fig. 1e).
Northeastern domain stations were more influenced by Arctic water and the
surface temperatures ranged between −2 and 1 ∘C (Fig. 1e). Surface
nutrient concentrations, particularly [NO3+NO2] and
[Si(OH)4], showed a broad range. The highest surface
[NO3+NO2] was observed in the southwestern fjords, between 2 and
>8µM, and the Atlantic station (∼3µM, Fig. 1a).
The surface concentrations at the remaining stations
were <0.5µM or near detection limits (Fig. 1a).
[Si(OH)4] was lower than [NO3+NO2] (i.e., Si:N<1)
among stations where [NO3+NO2] was >0.1µM.
At high [NO3+NO2] stations, the [Si(OH)4] ranged from
1.1–4.5 µM (Fig. 1b) but the range was lower among other stations
(0.4–1.1 µM, Fig. 1b). The bSiO2 concentration (proxy for diatom biomass, Fig. 1c)
was typically similar to, or lower than, surface [Si(OH)4]. The
highest surface [bSiO2] was observed in the southern stations
(Atlantic-influenced waters): ∼2–3 µmol Si L−1
(Fig. 1c). At most other stations the [bSiO2] was <1µmol Si L−1.
Among all stations and depths bSiO2 varied by a factor of
∼40 (does not include Hinlopen ice algae).

Primary productivity, measured at six stations at 5 m (approximately 50 %
of surface irradiance), varied by 2 orders of magnitude. The lowest primary
productivity rates were observed at the four stations with the lowest surface
[NO3+NO2] and ranged from 2 to 13 µgCL-1day-1;
at these stations [Chl a] ranged from 2.0 to 4.8 µg L−1 (Table 1,
Fig. 1d). The highest rates were measured at van Mijenfjorden and Bredjupet: 100±65 and 27±1µgCL-1day-1, respectively, and corresponded to high
[NO3+NO2] and low [Chl a] of 1.8 and 0.7 µg L−1,
respectively (Table 1, Fig. 1d).

3.2 Vertical profiles

As expected, most stations showed strong vertical gradients in nutrient
concentrations. Profiles in the southwestern region of Svalbard (van
Mijenfjorden, Bredjupet) had elevated [Si(OH)4], with little vertical
structure. Vertical [Si(OH)4] profiles among other stations showed
typical nutrient drawdown between the surface and ∼20m. At
these stations, surface [Si(OH)4] concentrations were typically
<1.5µM and subsurface values (to 20 m) ranged from 0.5 to 3.0 µM
(Fig. 2a). [NO3+NO2] exceeded [Si(OH)4] among all
depths at five stations (Fig. 2b), whereas in the remaining stations
[NO3+NO2] exceeded [Si(OH)4] (i.e., Si:N<1) at
depths >5m (Bellsund Hula), >20m (Erik
Eriksenstretet), and >27m (Polar Front). For these latter three
stations, [NO3+NO2] had a significant drawdown in surface
waters, but then increased with depth without a similar degree of vertical
enhancement in [Si(OH)4] (Fig. 2).

Figure 3Diatom abundance (a) and assemblage composition (b–d) in the
water column, and diatom export (e) and assemblage composition (f–h) within
sediment traps. Note – taxonomy information only shown for stations where
both water-column and sediment-trap data were available (see text for
species). Resting spores (e.g., Chaetoceros, Thalassiosira) were absent from the 40 m sediment traps;
thus, proportional abundances for spore-producing taxa are entirely for
vegetative cells. For (a), there are replicate diatom abundance
measurements (from separate hydrocasts) for the Polar Front station.

The [bSiO2] was typically highest at or near the surface, with a maximum of
∼2µmol Si L−1 (Fig. 2c). At the Bellsund Hula
and Erik Eriksenstretet stations, subsurface [bSiO2] maxima were
present (Fig. 2c; note – no surface data are available for van Mijenfjorden).
Among nonprofile stations, [bSiO2] was within the range observed among
vertical profiles except for the Hinlopen ice algae where ice, which was
melted at ambient air temperature on the vessel, had exceptionally high
[bSiO2] (Fig. 2c). The integrated bSiO2 between the
surface and 20 m (∫bSiO2) spanned over an order of magnitude, with a low at
Bredjupet (1.9 mmol Si m−2) and a high at Hornsunddjupet (42.4 mmol Si m−2, Table 1)
despite their proximity (∼50km).

Diatom abundance and taxonomy data were sampled at fewer stations, but the
vertical and spatial variability generally mirrored trends in [bSiO2].
In the surface waters of van Mijenfjorden and Hornsund, diatom abundances
ranged between 5×104 and 5×105cells L−1 in the upper 50 m (Fig. 3a).
However, within the same vertical layer at the Erik Eriksenstretet and
Polar Front (duplicate profiles) stations, diatom abundances were enhanced
by up to 2 orders of magnitude (4×104–4×107cells L−1,
Fig. 3a). When integrated to 40 m depth (∫Diatom), matching the
shallowest sediment-trap depth among the three stations reported (Fig. 3e–h),
diatom inventories also showed a variability of 2 orders of magnitude
as observed in ∫bSiO2. ∫Diatom was lowest at van
Mijenfjorden (7.67×109cells m−2) and highest at the Polar Front
station (527×109cells m−2, Table 1).

Diatom bSiO2 productivity, ρ, mirrored trends in biomass. Among
the profiles, rates generally varied from ρ<0.01 to 0.11 µmolSiL-1day-1 (Fig. 2d).
The ρ was highest in the
Atlantic station (Fig. 2d), which was expected given the higher bSiO2
(Fig. 2c). However, the rates in the Hinlopen ice algae were like those
quantified at Hornsunddjupet, ∼0.1µmolSiL-1day-1,
despite the ice algae station having an order of magnitude more
biomass. This suggests the Hinlopen ice algae were senescent or stressed and
a sizable portion of the measured bSiO2 was nonactive or detrital.
When integrated in the upper 20 m, ∫ρ ranged from
0.27 to 1.46 mmolSim-2day-1 (Table 1), which is a smaller
proportional range than observed in ∫Diatoms and ∫bSiO2. Overall, bSiO2-normalized rates (Vb) were low among
all stations and depths (<0.01 to 0.13 day−1). The
depth-weighted Vb, i.e., VAVE, had a narrower range of between
0.03 and 0.13 day−1. Thus, doubling times for bSiO2 in the upper
20 m
ranged between 5 and 23 days.

Figure 4Assessment of Si uptake limitation by available silicic acid
during ARCEx. (a) Eight-point kinetic experiments taken at four stations (legend
next to b). Data were fit to a Michaelis–Menten hyperbola using
SigmaPlot 12.3 software. (b) Enh. ratio profiles (i.e., Vb in
+18.0µM [Si(OH)4] treatment relative to Vb in the ambient
[Si(OH)4] treatment) at four stations.

The rate of diatom biogenic silica production was kinetically limited by
ambient [Si(OH)4] in 95 % of the samples examined. Full kinetic
experiments verified that Si uptake conformed to Michaelis–Menten kinetics
(Fig. 4a; adjusted R2 ranged 0.64–0.92 among experiments). The highest
Vmax was observed in the Atlantic station (0.36±0.02day−1),
which also had the highest ambient [Si(OH)4] among the full kinetic
experiments (1.4 µM). Vmax observed at Edgeøya and the Polar
Front was nearly identical (0.05±<0.01day−1 for both)
and was lowest in the Hinlopen ice diatoms (0.02±<0.01day−1). KS
constants had a narrower range, with a low of 0.8±0.3µM at the Polar Front and between 2.1 and 2.5 µM among the
other three stations. Among these full-kinetic experiments, the Enh ratio
ranged from 1.8 to 7.7, with the most intense [Si(OH)4] limitation of
uptake observed in the Hinlopen ice diatoms. For profiles where two-point
kinetic experiments were conducted, the same trends were observed (Fig. 4b).
The Enh ratio was similar among depths at Bellsund Hula (1.5–2.2),
Hornsunddjupet, and Bredjupet (3.4–5.4 for latter two stations, Fig. 4b). At
Erik Eriksenstretet, Enh ratios were more variable, ranging from 2.8 to 7.3 in
the upper 10 m, with no Enh effect (i.e., <1.08) observed at
20 m – this was the only sample and depth which showed no resolvable degree of
kinetic limitation for Si uptake.

Rates of bSiO2 and diatom export were variable. Among the three
sediment trap regions, bSiO2 export rates ranged from ∼4 to 10 mmolSim-2day-1
(Fig. 2e). These rates are significant and
represent up to 50 % of the ∫bSiO2 in the upper 20 m at van
Mijenfjorden (Table 1). For diatom cells, a similar degree of variability
was observed. Export at van Mijenfjorden ranged from 390 to 1500×106cellsm-2day-1,
similar to ranges at Hornsund (520–2800×106cellsm-2day-1) and Erik Eriksenstretet (510–860×106cellsm-2day-1,
Fig. 3e). The Atlantic station had significantly higher
diatom export (800–2300×106cellsm-2day-1) among all depths
in the upper 120 m (Fig. 3e). The bSiO2 and the export of diatom cells
were highly correlated (r=0.67, p<0.01, n=15; Spearman's
rho test). Among all stations, Fragilariopsis cylindrus had the highest contribution to diatom
export, and Thalassiosira species (e.g., T. antarctica, T. gravida,
T. hyalina, T. nordenskioeldii) were also important (Fig. 3f–h). In
Hornsund, Navicula (N. vanhoefenii, N. sp.) was an important genus for export (Fig. 3g), but this was not
observed elsewhere. Similarly, “Other diatom” groups were proportionally
important at Erik Eriksenstretet (Fig. 3h), as were Thalassiosira resting spores at the
Atlantic station (data not shown). Among all diatoms, the only groups which
were numerically important in both the water column and the sediment traps
were Fragilariopsis cylindrus and Thalassiosira species (Fig. 3b–d, f–h).

4.1 Diatom Si cycling relative to other systems

To our knowledge, this is the first report of bSiO2 production data of
the natural diatom community in this sector of the Arctic. Other studies
have reported ρ data in the subarctic Atlantic Ocean (Brown et al.,
2003; Kristiansen et al., 2000; Allen et al., 2005) ∼10–20∘
latitude south of our study region or in Baffin Bay (Hoppe
et al., 2018; Tremblay et al., 2002). However, the Hoppe et al. (2018) study
only includes ρ measured after a 24 h manipulation experiment and
only at one site and depth near the Clyde River just east of Nunavut
(Canada); no data are reported for the ambient conditions, and the
measurements from Tremblay et al. (2002) are based on net changes in
standing stocks instead of gross bSiO2 production. Banahan and Goering
(1986) report the only ρ to date in the southeastern Bering Sea;
however, Varela et al. (2013) recently reported that [Si(OH)4] in
surface waters (>5µM) is unlikely to be significantly
limiting to diatoms in any sector of the Bering, Chukchi, or Beaufort Sea
regions. Around Svalbard, some previous studies have examined other
Si-cycling components, including variability in bSiO2 in the water
column (Hodal et al., 2012) and sediments (Hulth et al., 1996), bSiO2
and diatom export (Lalande et al., 2016, 2013), or trends in
[Si(OH)4] (Anderson and Dryssen, 1981). The ρ measurements
presented here have no straightforward study for comparison; therefore, we
compare these to the previous high-latitude Atlantic data and to
well-studied sectors of the Southern Ocean.

During our study, ∫ρ in the Svalbard vicinity was low. In
the northeastern Atlantic between Iceland and Scotland, the reported ∫ρ
ranged between 6 and 166 mmolSim-2day-1 (Brown et al., 2003; Allen
et al., 2005). These rates are significantly higher than at our four profile
stations (Table 1), and the degree of difference does not appear to be
driven by differences in integration depth (compared to our study, Table 1).
Given the higher [Si(OH)4] in the southern region of the Atlantic
subpolar gyre (Hátún et al., 2017), the maximum achievable
∫ρ may vary with latitude. While our profile sampling was
opportunistic, it appears we sampled some stations with significant diatom
biomass (high ∫bSiO2), but the corresponding production
rates (∫ρ) were low, with estimated doubling times on the
order of 11–23 days. This suggests these high-biomass stations may have
been near, or past, peak bloom conditions (Fig. 2a, b) and the seasonal
timing is consistent with regional field and modeling studies inferring
diatom bloom dynamics from Chl a trends (Wassmann et al., 2010; Oziel et al.,
2017). Kristiansen et al. (2000) reported ρ in Oslofjorden during the
late winter (February–March) ranging from 0.03 to 2.0 µmolSiL-1day-1,
over nine sampling periods with corresponding Vb of between <0.01 and 0.28 day−1; however, this system has a higher Si(OH)4 supply
and surface concentration at the start of the bloom period (>6µM),
approximately 50 % higher than the highest surface
concentrations observed during our study (Fig. 2a). The specific rates
observed in our study fall within the lower values reported by Kristiansen
et al. (2000), which may be explained by the reduced uptake from lower
[Si(OH)4] (e.g., Fig. 4).

The Southern Ocean is one of the most globally significant regions for
production of bSiO2. The surface [Si(OH)4] and
[NO3+NO2] are among the highest in the ocean and the source
waters usually have >50 % excess Si(OH)4 relative to
nitrate (Brzezinski et al., 2002). Thus, exceptional Si(OH)4 drawdown
relative to nitrate is required for diatom biomass yield to be limited by Si
in this region. The mean ∫ρ in sectors of the Southern
Ocean are variable. In the Weddell Sea, winter rates range between 2.0 and 3.2 mmolSim-2day-1
in the seasonal ice zone (Leynaert et al., 1993).
Within the sub-Antarctic zone, rates averaged 1.1 and 4.8 mmolSim-2day-1
in the summer and spring, respectively (Fripiat et al., 2011). At
the terminus of diatom blooms in the sub-Antarctic and polar frontal zone,
rates can be lower, e.g., 0.1–0.3 mmolSim-2day-1 (Fripiat et al.,
2011); such values are similar to the range observed during our study,
especially since these Southern Ocean studies integrated ∫ρ
deeper than 40 m (e.g., 50–100 m). Brzezinski et al. (2001) reported average
∫ρ of ∼25mmolSim-2day-1 (integrated
from surface to 80–120 m) during intense blooms in the seasonal ice
zone, which propagated south of the Antarctic polar front. But despite the massive
diatom bSiO2 accumulating in these blooms, VAVE generally ranged
between 0.05 and 0.15 day−1 (Brzezinski et al., 2001). Given the
order-of-magnitude difference in [Si(OH)4] and ∫ρ
between the Arctic and Southern Ocean, the similar VAVE in both regions
may be more reflective of thermal effects on diatom growth rate, since Si
uptake and diatom growth rates are tightly coupled, or of a significant
accumulation of detrital bSiO2 (i.e., diatom fragments) in the Southern
Ocean, where low temperatures reduce bSiO2 remineralization rates
(Bidle et al., 2002).

4.2 Potential for silicon limitation of diatom productivity

Suboptimal silicon availability affects the rate of diatom bSiO2
production and can limit their growth. For diatoms in Svalbard and the
broader region of the subpolar and polar European Atlantic, both
[Si(OH)4] and its availability relative to N appear to be suboptimal
for creating intense diatom blooms, such as those occurring in the Southern
Ocean. Yet, the Arctic spring bloom is consistently dominated by diatoms or
Phaeocystis (Degerlund and Eilertsen, 2010), which suggests some level of adaptation
for diatoms to the low [Si(OH)4] environment. Stoichiometry of silicon
availability relative to nitrate can help diagnose Si limitation; the most
widely accepted diatom Si:N ratio is ∼1 based on temperate
and low-latitude clones (Brzezinski, 1985). The average Si:N ratio for two
polar diatom clones (silicic acid and iron replete) reported in Takeda
(1998) was 0.96±0.24 (SE). A more recent culture study by Lomas and Krause (2018), reported Si:N for 11 polar diatom clones grown at
2 ∘C among exponential and stationary growth phases, in both replete and N-limiting nutrient conditions; these authors observed Si:N among
all clones, treatments, and nutrient conditions (>150 data
points) was 1.7±0.10 (SE).

The silicon kinetic data provide clarity for interpreting Si and N nutrient
drawdown. Diatoms have an r-selected ecological strategy and are typically
the first phytoplankton group to bloom in this region under stratified
conditions (Reigstad et al., 2002). The 1.7 Si:N from Lomas and Krause (2018)
for nutrient-replete polar diatoms suggests they consume 70 % more
Si relative to N. However, under kinetic limitation, diatoms have long been
inferred to reduce Si per cell in culture to avoid growth limitation
(Paasche, 1973) – this was recently observed directly in the field for the
first time (McNair et al., 2018). Given the clear kinetic limitation
observed during ARCEx (Fig. 4), this likely reduced the diatom Si:N ratio
to closer to the canonical 1:1 ratio. Thus, the kinetic limitation in this
region may result in N and Si being consumed in near-equal amounts (i.e.,
Si:N∼1) and previous inferences of diatom processes based on
1:1Si:N drawdown appear valid.

Figure 5Diagnosis of potential silicon limitation for diatom production
during ARCEx. (a) Nitrate + nitrite drawdown as a function of dissolved
silicic acid. (b) The ratio of Vb at ambient [Si(OH)4] to Vmax
versus dissolved silicic acid. In both panels, linear regressions were done
using a model II reduced major axis method; for panel (b) the regression line
does not include the Hornsunddjupet station (open circles). For comparison,
the same relationship for the Sargasso Sea in the North Atlantic subtropical
gyre is shown, as synthesized in Krause et al. (2012).

Nutrient relationships support the potential for silicon to be a controlling
factor of regional diatom productivity. When plotting [NO3+NO2]
as a function of [Si(OH)4] (Fig. 5a) a few trends emerge. (1) The slope
of the linear regression relationship (2.5±0.1mol N (mol Si)−1)
denotes that NO3+NO2 is consumed at over twice the
rate per unit Si(OH)4. (2) Given that the source water
[NO3+NO2] concentration is only roughly twice that of
[Si(OH)4], a 2.5 drawdown ratio would predict NO3+NO2 to be
depleted before Si(OH)4. The latter observation suggests that
Si(OH)4 could be the yield-limiting nutrient for diatoms during a
spring bloom period only if they dominate the phytoplankton assemblage and
consume Si:N in ratios >1, e.g., 1.7 as reported by Lomas and Krause (2018).
Field data demonstrate interannual variability. Nitrate and
silicic acid drawdown within the upper 50 m during the spring season
(1980–1984) was discussed by Rey et al. (1987) who suggested apparent
nitrate limitation (1980, 1981) and silicic acid limitation (1983, 1984).
The Reigstad et al. (2002) analysis of nitrate and silicic acid drawdown in
the central Barents Sea shows similarities to ARCEx in that the diatom
assemblage could only draw down [Si(OH)4] to ∼1µM
(May 1998) and ∼0.5µM (July 1999). These authors
suggest that physical effects on phytoplankton explain the variability,
where diatoms dominate in shallow mixed waters as opposed to Phaeocystis pouchetii dominating in
deeper mixed waters. Clearly, interannual and local differences in mixing,
which may favor Phaeocystis pouchetii over diatoms (Reigstad et al., 2002), can affect the
assemblage and nutrient drawdown trajectory (e.g., see points with high
[Si(OH)4] and [NO3+NO2] close to detection limit, Fig. 5a);
therefore, diagnosis of whether Si limits diatom production should be
accompanied by additional analyses.

When considering the European sector of the Arctic and sub-Arctic between
60 and 80∘ N, there is compelling evidence that ambient
[Si(OH)4] limits the rate of diatom bSiO2 production. During
ARCEx, the relationship between Vb and [Si(OH)4] supports
that Si regulates diatom productivity to some degree (Fig. 4). Our kinetic
data demonstrate that in three of four experiments KS was
∼2.0µM, but in the Polar Front the KS was lower
∼0.8µM. These data are consistent with community
kinetic experiments reported in Oslofjorden where KS and Vmax
were between 1.7 and 11.5 µM and between 0.16 and 0.64 day−1, respectively, with
the lowest Vmax observed during the declining diatom bloom (Kristiansen
et al., 2000). These authors concluded that silicon ultimately controlled
diatom productivity during this bloom (Kristiansen et al., 2001). In the
only other kinetic experiments reported in the northeast Atlantic, Brown et
al. (2003) and Allen et al. (2005) observed linear responses in Vb
between ambient and 5 µM [Si(OH)4], which suggests uptake did
not show any degree of saturation at this concentration (Note: the single
experiment reported in Allen et al. (2005) is one of four experiments
originally reported in Brown et al., 2003). These field-based KS
values are considerably higher than parameters used in Barents Sea models,
e.g., 0.5 µM (Slagstad and Støle-Hansen, 1991) and 0.05 µM
(Wassmann et al., 2006), which reflect the high-efficiency Si uptake reported
for cultures (Paasche, 1975). Fitting a regression to the VbVmax-1 as a function of [Si(OH)4] (line shown in Fig. 5b)
suggests that 2.8 µM is the best constrained half-saturation
concentration (i.e., concentration where VbVmax-1=0.5)
for the regional assemblage. This empirical value excludes the
Hornsunddjupet assemblage (white symbols, Fig. 5b), and their inclusion
decreases this aggregated half-saturation to 2.3 µM. Unlike diatoms
in the North Atlantic Subtropical Gyre, e.g., the Sargasso Sea (Krause et al.,
2012), regional diatoms do not appear to be well adapted for maintaining VbVmax-1>0.5 at low [Si(OH)4]. Instead, diatoms
during the spring season appear to be best adapted for concentrations
exceeding 2.3 µM. It is plausible that as [Si(OH)4] is depleted,
diatoms may slow growth from severe limitation of Si uptake (Fig. 5b)
and/or biomass yield (i.e., stock of diatom bSiO2 far exceeds
Si(OH)4).

To avoid growth limitation under conditions of kinetic limitation (i.e.,
suboptimal [Si(OH)4]), diatoms can reduce their silicon per cell. A
guideline from culture work is that diatoms can alter their silicon per cell
by a factor of 4 (Martin-Jézéquel et al., 2000). Thus, when
uptake is reduced to <25 % of Vmax (i.e., concentration which
promotes uptake at half the half-saturation level) diatoms must slow growth
to take up enough Si to produce a new cell. Using the empirical
half-saturation constant range (2.3–2.8 µM), calculated from
Fig. 5b
and using Eq. (1) to solve for the concentrations where VbVmax-1≤0.25 (Vmax is a constant), suggests that when
[Si(OH)4] is below 0.3–0.8 µM, the degree of kinetic limitation
could force diatoms to slow growth in response. This type of limitation
could occur even if diatom bSiO2 stock was not sufficiently high to
induce yield limitation, e.g., it could not deplete all Si(OH)4 from the
assemblage undergoing one division. Such a range is lower than the common
interpretation of the Egge and Aksnes (1992) data set showing diatoms may be
outcompeted by flagellates when [Si(OH)4]<2µM, a
value which is more reflective of an ecological niche opposed to a
physiological threshold, as has been purported in numerous citations of these
data. At these inferred limiting [Si(OH)4] there would be up to 0.8 µM
[NO3+NO2] remaining (Fig. 5a), which would allow
nonsiliceous phytoplankton to draw down the remaining N. Therefore, under
shallow stratified conditions which favor diatoms over Phaeocystis (sensu Reigstad et
al.,
2002), [Si(OH)4] may regulate regional diatom productivity through
either yield- or severe-kinetic limitation. This provides the most direct
assessment to date, supporting the general ideas proposed for Si regulation
of regional diatom productivity (Rey, 2012; Rey et al., 1987; Reigstad et
al., 2002).

4.3 Diatom contribution to primary production

Among the six sites with paired PP and ρ measurements, the bloom phase
can be inferred from the magnitude of nutrient drawdown, [Chl a], PP, and
pCO2 (data not shown). Bredjupet appeared to be a pre-bloom station
given the high surface nutrient concentrations, while the van Mijenfjorden
station appeared to be in an early bloom phase based on relative high
nutrients and moderate [Chl a]. The Erik Eriksenstretet station represented a
peak bloom condition, whereas assemblages at Hornsunddjupet and Edgeøya
appeared to be postbloom and in a stage of decline. The Polar Front station
represented the end or late-phase bloom condition; however, at this station
Phaeocystis was abundant (data not shown), suggesting it may have dominated the bloom
dynamics instead of diatoms.

The diatom contribution to PP was highly variable. Among the stations with
high [NO3+NO2] (van Mijenfjorden, Bredjupet) the diatom
contribution to PP (e.g., Eq. 2) was low, at 2 %–3 %. At two stations,
Hornsunddjupet and the Polar Front, the diatom contribution to PP increased
to 25 %–30 %. In the Edgeøya and Erik Eriksenstretet stations, diatoms
accounted for a majority or all of PP, at 70 % and 180 %, respectively.
Given that diatoms can reduce their cellular Si in response to kinetic
limitation (Paasche, 1973; McNair et al., 2018), the Si:C ratio of 0.25
based on nutrient-replete polar diatoms in culture may systematically
underestimate diatom contribution to PP using our approach. For example, if
kinetic limitation reduced Si per cell by 50 % (i.e., VbVmax-1≈0.50, Fig. 5b) but did not affect cellular C,
then the Si:C ratio would be 0.13 (i.e., temperate Si:C diatom value), and
nearly all the calculated diatom contributions would double. Considering the
degree of kinetic limitation at most stations (Fig. 5b), this suggests our
estimates are conservative except at Erik Eriksenstretet. The unrealistic
value at Erik Eriksenstretet underscores the issue with the Si:C ratio
(Eq. 2); however, adjusting Si:C downward would increase the diatom contribution.
Thus, there may have been lower C per cell for diatoms at this station due
to other factors associated with the phase of the bloom and/or the different
assemblage; e.g., Porosira glacialis was more abundant at this station and has a large vacuole which
could lower C content, thereby increasing Si:C (data not shown).

Clearly, diatoms can play a significant role in local productivity, but
these data demonstrate a “boom and bust” nature. At stations at or near
peak bloom levels (e.g., Edgeøya, Erik Eriksenstretet), diatoms could
account for nearly all primary production. However, they may also contribute
an insignificant percentage of primary production prior to the onset of the
bloom (e.g., van Mijenfjorden, Bredjupet). But even when physical conditions
may favor Phaeocystis blooms, diatoms appear to be significant contributors to primary
production (Polar Front station). In such a situation, N would be predicted
to be the limiting nutrient as it will be consumed by both Phaeocystis and diatoms
whereas Si will only be consumed by the latter.

In the European Arctic, shifts in summer-period phytoplankton communities
away from diatom-dominated conditions have been observed in numerous
studies. One of the most important observations has been the increasing abundances of
Phaeocystis in ice-edge (Lasternas and Agustí, 2010) or under-ice blooms (Assmy et
al., 2017). These changes have corresponded with larger-scale shifts in the
export of diatoms to depth in the Fram Straight (Nöthig et al., 2015;
Lalande et al., 2013; Bauerfeind et al., 2009). The timing of these shifts,
e.g., mid-2000s, correspond with the broader regional reduction in winter
mixed-layer [Si(OH)4] concurrent with the shift to negative gyre-index
state in the latter half of the decade (Hátún et al., 2017). With a
reduction in pre-bloom Si(OH)4 supply, diatoms may run into limitation
sooner during the bloom cycle and thus leave more residual nitrate for
nondiatom phytoplankton. Given the highly variable contribution of diatoms
to primary productivity, resolving a climate change or natural physical
oscillation signal will be challenging. A similar conclusion about detecting
a climate-change signal was made in the eastern Bering Sea by Lomas et al. (2012) given the natural variability in primary production.

4.4 Diatoms and export

The bSiO2 export rates observed during ARCEx were significant relative
to the standing stocks. At van Mijenfjorden, the rate of export in the upper
40 m represented 39 % of the ∫bSiO2 standing stock (23.3 mmol Si m−2,
integral of data in Fig. 2c) in the same vertical layer.
This quantity was much higher than at Erik Eriksenstretet, where the
40 m
export rate was <11 % of the ∫bSiO2 in the upper
water column (note: no samples were taken deeper than 20 m; thus, additional
bSiO2 between 20 and 40 m would lower the 11 % estimate). Given that the
van Mijenfjorden site was located within shallow fjord waters (bottom depth
approximately 60 m), such a high proportion export relative to standing
stock may suggest either lateral focusing processes (e.g., discussed by
DeMaster, 2002) and/or resuspension of sediment bSiO2 into the water
and resettlement. The rate of bSiO2 export among all export and
production stations was also at least a factor of 4 higher than
∫ρ in the upper 20 m (Table 1). It is likely that some
fraction of ∫ρ was missed due to lack of sampling between
20 and 40 m, but with a less light at these depths, it is unlikely systematic
underestimates of ρ caused the disparity. Given the deeper water at
the Erik Eriksenstretet and Atlantic stations, such high bSiO2 export
may be driven by previously high ρ and bSiO2 standing stock
that accumulated in the overlying waters or, given the dynamic circulation in the
region, this signal may have been laterally advected to these station
locations.

Relative to previous studies, the bSiO2 export rates were also high.
During May 2012 in Kongsfjorden, Lalande et al. (2016) reported bSiO2
export rates between 0.2 and 1.3 mmolSim-2day-1 in the upper 100 m. A similar range was observed by Lalande et al. (2013) in the eastern Fram
Strait using moored sediment traps (2002–2008) collecting at depths between
180 and 280 m. Lalande et al. (2013) concluded that, despite warm anomaly
conditions, pulses of bSiO2 export were positively correlated to the
presence of ice in the overlying waters, which stratifies the water and helps
initiate a diatom bloom. However, if the light was insufficient to stimulate
a bloom, Lalande et al. (2013) suggested much of the pulse of bSiO2
exported to depth may have originated in the ice and sank during melting.
Indeed, the low Vb (<0.01day−1) observed at the Hinlopen
station (ice algae), despite the moderate ρ measured (0.12 µmolSiL-1day-1), suggests that most of the ice-associated bSiO2
was detrital and not associated with living diatoms. Thus, the recent ice
retreat observed prior to the ARCEx cruise was a potential source of such
high bSiO2 export to depth despite the considerably lower ∫ρ in the upper 20 m.

Among the groups examined, the most important diatom genera for standing
stock and export were Thalassiosira and Fragilariopsis, suggesting these groups are important drivers
of bulk bSiO2 fluxes. Given the large-size and chain-forming life
histories for the dominant species within each genus, it is likely that
their dominance in the trap abundances helps explain the high correlation (r=0.67, p<0.01; Spearman's rho test) between bSiO2 and
diatom export. Given this degree of correlation, it would be expected that
both bSiO2 and diatom export would be similarly enhanced relative to
previous studies; however, this was not observed.

Comparing the magnitude of bulk bSiO2 export and the export of diatom
cells suggests significant food web repackaging occurred. The export of
diatom cells in Kongsfjorden (Lalande et al., 2016) were similar to or a
factor of 3 lower than rates quantified during ARCEx (Table 1, Fig. 3e),
whereas bSiO2 export during ARCEx was over an order of magnitude higher
than bSiO2 export in Kongsfjorden. One possible explanation for the
higher degree of bSiO2 export enhancement, relative to diatom-cell
export, between studies is that more exported material during ARCEx was
repackaged and modified in the food web. For instance, in Erik
Eriksenstretet gel traps confirm the presence of aggregates and
mesozooplankton fecal pellets (Ingrid Wiedmann, data not shown), and in van
Mijenfjorden detrital particles and sediment material were most prominent on
the gel traps opposed to clearly recognizable material (e.g., diatom valves).
This repackaging is consistent with previous observation in the Barents Sea
showing high potential for copepod fecal pellets to be exported in the Polar
Front and Arctic-influenced regions during spring (Wexels Riser et al.,
2002). It also supports the general ideas for the importance of diatom organic
matter in fueling secondary production regionally during this season
(Degerlund and Eilertsen, 2010, and references therein).

This is the first regional data set with contemporaneous measurements of
diatom bSiO2 standing stock, production, and export and assessment of
kinetic limitation by [Si(OH)4] in the European Arctic. Among stations
and depths there was widespread limitation of diatom bSiO2 production
rates by ambient [Si(OH)4] during spring-bloom conditions. The kinetic
parameters for diatom Si uptake (e.g., KS) quantified in our study are
significantly higher than rates used in regional models and quantified in
polar diatom cultures; therefore, these data will help future modeling
efforts better simulate diatom–Si dynamics. Given the trajectories of Si and
N consumption, diatom-dominated blooms (vs. Phaeocystis-dominated) could deplete
Si(OH)4 prior to nitrate (yield limitation); and at some stations, the
degree of kinetic limitation by ambient [Si(OH)4] could have resulted
in diatom growth being slowed. Diatom contribution to PP was highly
variable, ranging from <10 % to ∼100 % depending
on the bloom stage; but even when Phaeocystis appeared to be favored, diatoms still had
a significant (∼25 %) contribution to PP. While there was
agreement with previous regional studies regarding the export rate of diatom
cells, we observed significantly elevated bSiO2 export. Such a
discrepancy can be resolved if a higher fraction of the diatom material
exported during our study was modified by zooplankton grazers, relative to
previous studies, or if much of this bSiO2 was derived from melting ice
and/or advection.

JWK, CMD, and SA conceived and designed the study and conducted analysis. JWK, CMD,
IAM, PA, MFM, IW, and SA conducted the fieldwork. PW and SK conducted analysis.
All coauthors contributed to the writing of the paper, led by JWK.

The authors thank the science team and crew of the RV Helmer Hanssen.
We also thank Sigrid Øygarden, Emma Kube, Angelika Renner, Daniel Vogedes, Hanne Foshaug, Sydney Acton, David Wiik,
Britt Vaaja, and William Dobbins for logistic support. We also thank two anonymous reviewers for constructive
feedback. Primary data analysis was
supported by the Dauphin Island Sea Lab. Vessel time, ancillary data, and Ingrid Wiedmann's and Paul Wassmann's
contribution was supported by ARCEx, funded by
industry partners and the Research Council of Norway (project no. 228107).
Philipp Assmy was supported by the Research Council of Norway (project
no. 244646). Philipp Assmy and Mar Fernández-Méndez were funded by
Norwegian Ministries of Foreign Affairs and Climate and Environment through
the Arktis 2030 programme (project ID Arctic). Jeffrey W. Krause, Carlos M. Duarte, and
Susana Agustí were supported by internal funding sources at their respective
institutions.

Tremblay, J.-É. and Gagnon, J.: The effects of irradiance and nutrient
supply on the productivity of Arctic waters: a perspective on climate
change, in: Influence of climate change on the changing Arctic and
sub-Arctic conditions, Springer, Dordrecht, 73–93, 2009.

Diatoms can dominate the Arctic Ocean spring bloom, the key annual event for regional food webs. Diatom growth requires silicon and this nutrient has been declining in the European Arctic. This study communicates an unprecedented combination of silicon-cycling measurements around Svalbard during the spring and shows that dissolved silicon can limit diatom production. These results suggest an important coupling of silicon and carbon cycling during the spring bloom in the European Arctic.